NTC thermistors are thermistors which have a negative resistance-temperature coefficient, that is, NTC thermistors have relatively high resistance at room temperature which decreases as the temperature is increased. Therefore, NTC thermistors are used as a temperature sensor or in a temperature compensation circuit, for example.
Generally, the resistance-temperature characteristic of an NTC thermistor is not linear. When an NTC thermistor is utilized as the above described temperature sensor or in a temperature compensation circuit, a more linear resistance-temperature characteristic is preferred so that the response to small temperature changes is linear. The conventional method of obtaining a preferred resistance-temperature characteristic by using a fixed resistor or an IC device, for example, in the circuit in which an NTC thermistor is adopted is well known. However, that method requires a complicated circuit and results in higher cost.
Japanese Unexamined Patent Application Publication No. 2003-272904 (Patent Document 1) discloses an NTC thermistor having a linearized resistance-temperature characteristic by laminating two kinds of NTC thermistor materials in one body. Japanese Unexamined Patent Application Publication No. 2002-231508 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. 2-189901 (Patent Document 3) disclose a similar method. In the Patent Document 2, a thermistor having linear resistance-temperature characteristics over a wide temperature range by uniting a first and a second thermistor which are different from each other in the characteristic into one body so as to be equivalent in parallel without causing reaction therebetween is disclosed. In the Patent Document 3, an NTC thermistor of linearized resistance-temperature characteristic by laminating and forming one body with two or more kinds of resistor layers which have different resistance-temperature characteristics is disclosed.
All of the methods disclosed in the Patent Documents 1 to 3 is conceived to obtain a linearized resistance-temperature characteristic by laminating or uniting ceramic layers of at least two kinds of thermistor material which have a different negative resistance-temperature characteristic each other.
According to the method disclosed in the Patent Documents 1 to 3, problems exists in that at least two kinds of thermistor material must be prepared and, furthermore, selecting material which make a linearized resistance-temperature characteristic is difficult. Moreover, since ceramic layers composed of at least two kinds of thermistor material are united, undesired characteristic change or cracking due to a weak united interface tends to occur from elemental diffusion, a difference of contraction percentage or a difference of linear coefficient of expansion, for example, at firing.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-272904    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-231508    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2-189901